This research study is designed to learn about the effects upon bone density in children undergoing stem cell or bone marrow transplan. Specifically, the aims of this study are: 1) To determine the prevalence of decreased bone mineral density (BMD) in children undergoing allogeneic bone marrow transplant (BMT); 2) To correlate the BMD withthe duration and dose of steroid and cyclosporine use (which is correlated with duration of chronic graft vs. host disease (GVHD); 3) To measure the changes in bone mineralization over time and post-steroid/cyclosporine use; 4) To assess the incidence of avascular necrosis and fractures in these patients compared to age and gender-specific fracture incidence reference data. Currently there are seven patients enrolled: three females (one black, two caucasion), and four male (all caucasion). A total of nine patients have consented to this study, but two were lost to attrition (one death due to multi system organ failure, and one due to ineffective sedation). Patients will continue to be enrolled over the next four years and we anticipate enrolling approximately 100 patients. If this study confirms that bone mineral density is diminished in a sample population of children post bone marrow transplant, and either the bone mineral density does not recover or the incidence of bone complications is increased, a randomized clinical trial of therapy for optimizing bone mass in children may be warranted.